Do some cleanup in ah_massage_headers().
- Declare global array ipseczeroes containing zeroes constant.
- The proto parameter contains the address family, so call it af.
- Remove an unused if block, just keep the else.
- If m_copyback(M_NOWAIT) fails, return with error instead of working
with an inconsistent mbuf.
- ip6_nxt is u_int8_t, no need to clear the high bits.
- The offset and next protocol are advanced for all extension
headers, move it after the switch.
- ah_massage_headers() returns an errno, call the variable error.
OK procter@